1. Field of the Invention
This invention provides a means for adjusting the volumetric capacity of a pressurized-fluid seismic sound source, without changing the external dimensions thereof. The subject matter might be found in Class 181/120.
2. Discussion of Related Art
Pressurized-fluid seismic sound sources are extensively used in marine seismic exploration for liquid and gaseous hydrocarbons. To a lesser extent they are used for special types of land surveys.
The fluid used in such sources may be a liquid under pressure, such as water, or a compressed gas such as air. By way of example and convenience, but not by way of limitation, I shall describe my invention in terms of air guns.
In its simplest form, an air gun consists of a firing chamber for containing a volume of air compressed from 500 to more than 6000 pounds per square inch (psi). The firing chamber is provided with one or more air exhaust ports that may be closed by a sliding, differential-area valve. The valve may be a piston, a sliding shuttle, or an internal or external sliding sleeve valve. In operation, the valve is held closed by control-air pressure in a valve-control chamber. The area of the valve face that is exposed to the control-air pressure is greater than the area of the opposite valve face that is exposed to the air pressure in the firing chamber. The greater force applied by the control-air pressure holds the valve closed. Upon command, a solenoid valve bleeds off the control-air pressure. When the balance of forces is upset across the two faces of the valve, the air pressure in the firing chamber causes the valve to abruptly open. Some of the air in the firing chamber is impulsively released through the exhaust ports into the water to create a shock wave which then degenerates into an acoustic wave.
The air released from the air gun creates a pulsating air bubble in the water. The periodicity of the bubble depends upon the static water head, the air pressure in the firing chamber and the volume of air released. The pulsating air bubble wreaks havoc with the resulting seismic signals because each expansion of the bubble acts like a new seismic source. Although computer-aided data processing can de-bubble the recorded seismic signals, it is preferable to suppress or eliminate the bubble effects ab initio in the field.
In accordance with modern practice, arrays of air guns are employed to maximize the primary-to-bubble amplitude ratio. In the array, the respective air guns are characterized by firing chambers having different volumes. Since the periodicity of the bubble is a function of the volume of air released, by properly tuning the individual guns of the array, the bubble pulses generated by the guns, tend to cancel by destructive interference. The firing-chamber volumes of the guns in a single array may range from a few cubic inches to several hundred cubic inches. In an array of 15 to 20 guns, the combined volumetric capacity may be as much as 5000 in.sup.3.
It is preferable, of course, that, regardless of the volumetric capacity of the respective firing chambers, the basic operating parts of the air guns of an array be interchangeable and of uniform size, e.g. valve, control-air chamber, triggering solenoids and the like. To change firing-chamber volume, it should only be necessary to change the firing chamber itself.
Some air-gun designs are readily adaptable to that desideratum. See for example U.S. Pat. No. 4,114,723, which is incorporated herein by reference as an example of one type of air gun. In that patent, the firing chamber consists of an elongated, cylindrical casing, capped at one end. The other end is screwed to a plug that includes the internal sleeve valve and the valve-control assembly. Although not specifically taught in that patent, it is a simple matter to change the air-gun volume by replacing an existing elongated casing by one of a different length. The dimensions of the operating parts of the gun remain unchanged. Since the mechanical operating parts of the gun are interchangeable regardless of gun volume, the spare-parts inventory is minimal.
Another style of air gun has recently become popular which is exemplified by U.S. Pat. No. 4,623,033, issued 11/18/86, which is also incorporated herein by reference. That air gun employs an externally mounted sliding sleeve valve or shuttle and a partially externally-mounted valve-control chamber. The firing-chamber portion of that air gun includes a supporting center post that is integral with the body of the gun. An external sleeve surrounds and encloses the center post assembly.
It is not possible with the '033 gun to merely change the dimensions of the firing chamber to change its volume. It is necessary to physically re-scale the entire gun assembly including the external shuttle valve, the valve-control mechanism and the sleeve. For that reason a large stock of complete guns, all of different physical sizes, along with sets of differently-sized spare parts, must be maintained. That requirement poses a horrendous logistics problem.
There is a need for a standard-sized air gun assembly, the firing-chamber volume of which is readily adjustable, without affecting the interchangeability of the mechanical operating parts.